reentrant : Java Glossary

All Java methods are automatically reentrant. It means that
several threads can be executing the same method at once, each with its own copy of
the local variables. In the olden days of IBM (International Business Machines)
360 mainframes, which had no hardware stacks, reentrancy did not come out in the
wash and you had to take special time-consuming measures to make just the methods
reentrant that needed it.